1. Field of the Invention
The present invention relates to a method of forming microcapsules containing hydrophobic oil droplets therein and having wall films of urea-formaldehyde resin therearound. More specifically, the present invention relates to a method of forming microcapsules for the purpose of improving the function of the microcapsules wherein carboxylated polyvinyl alcohol is present during the polymerization reaction of urea and formaldehyde whereby the urea-formaldehyde capsule walls are produced around individual droplets of the hydrophobic oily solution dispersed in an aqueous phase.
2. Description of the Prior Art
Heretofore, various macromolecular electrolytes have been tried during the formation of microcapsule walls by polymerizing urea and formaldehyde around a previously dispersed hydrophobic oily solution. These electrolytes function to prevent coagulation of the microcapsules as well as as a binder and an emulsifying oil. For instance, ethylene-maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer and polyacrylic acid have been employed as macromolecular electrolytes, as disclosed in U.S. Pat. No. 4,001,140. However, microcapsule wall films produced utilizing these macromolecular electrolytes do not have sufficient heat resistance and mechanical strength, and/or exhibit high permeability. Consequently, the use of oil-containing microcapsules produced using the above-described macromolecular electrolytes in pressure sensitive copying sheets has been accompanied by the defects described below;
(a) Deterioration in the permeability of the microcapsule wall film caused by heat resulting in a decrease in the coloring ability of the pressure sensitive copying sheets when allowed to stand in high temperature atmosphere. PA1 (b) When a plurality of pressure sensitive copying sheets are wetted with water in a stacked condition, intense coloration of the color developer sheets or the capsule sheets occurs due to the high permeability of the microcapsule wall films. PA1 (1) preparing an aqueous solution of a carboxylated polyvinyl alcohol and urea; PA1 (2) emulsifying and dispersing a hydrophobic oily solution substantially insoluble in the aqueous solution prepared in step (1) which becomes the core material in the aqueous solution (1); PA1 (3) admixing formaldehyde; and PA1 (4) allowing urea and formaldehyde to copolymerize in the resulting aqueous system with stirring.